The first thing to do is know your enemy and know yourself first. If you know this, everything else is obvious. Always be observant of things around you. This includes objects. Always listen to people around you and your reactions. Always be willing to listen. Think of things with a three track mind. Be able to say one thing and think another while in perfect control of your body. When talking, never be remembered for important facts but for the funny remarks as well. Never be obvious that you are manipulating the people through every action of your body, glance and speaking patterns. If caught, things could get ugly for you. Get an understanding of the situation before you do. When the idea of success happens to your mind. Make certain you don't get too cocky. This rule applies: cocky, stupid, lazy, and dead. Get too cocky because of so many successes. Stupidity happens because you think you can do anything. Laziness is the result and sloppy mishandling of a situation occurs through reactions. Death happens afterward in some manner. Thus to be an envoy, one must handle any situation without cockiness.
The second thing is to say an oath of non destructive acts to solve the problem. After that, learn yourself by observing yourself and memorize everything that you do. Analyze during the night. When you feel that you know yourself you can start studying people. Know people through the actions that they do and by what they say. Thus, the art of language is learned in this manner. When one knows this language, learn body language. This takes a little longer. When the words a person speak and the body language they show are understood then you ready to be an envoy. Now try to find how to be an analyst in yourself. Understand psychology as it will be one of your weapons. Understanding the bargaining and barter ideal also helps. Now you have the abilty to be an envoy.
Finally, the idea of practicing is necessary. Where one travels, one can find trouble is a hindu saying. Thus, practice it on the arguments you find. Find trouble by going somewhere and diffuse the situation by using an envoys tricks of manipulation through non destructive actions. Keep doing this until you are good enough to face a criminal without flinching. Thus, you are on your way to becoming a master. A envoy could be considered a translator of actions and events. Try not to be a target if you can. It always keeps you on your feet.